In current Direct Sequence Spread-Spectrum (DSSS) high-speed data standards and proposals, multiple orthogonal code channels are used to achieve very high data rates. In these systems, user data is either Time Division Multiplexed (TDM) and/or Code Division Multiplexed (CDM) using multiple code channels. Due to the bursty nature of most data traffic, for example, Hyper Text Transfer Protocol (HTTP) traffic, overhead channels are usually used to direct the packet data traffic on the forward link (down link).
The mobile users to whom the data is scheduled for from the Base Transceiver System (BTS) will be directed to data traffic channels via these overhead channels, while the mobile users to whom there is no data scheduled, will cease decoding the data channels after decoding the overhead channels. This approach can efficiently make use of the orthogonal code space and reduce interference.
In general, there are two types of overhead channels in CDMA high-speed data systems, namely, common control channels and dedicated control channels. Common control channels, such as those used in the 1xEVDV (also referred to as CDMA 2000) specification have the advantage of saving available codes, but have the disadvantage of introducing higher interference because they are not efficiently power controlled. Overhead control channels are usually low speed and thus are usually spread by longer orthogonal codes to provide higher processing gain. The data channels are typically spread using shorter orthogonal codes due to the higher data rates used. When dedicated control channels are used together with multicode data traffic channels via CDM in the same time frame, demodulation and processing of the overhead control channels requires dedicated hardware (and/or software) other than hardware (and/or software) allocated for demodulation and processing of the data channels. Namely, one demodulator for each overhead control channel is required to implement some of these prior art systems.
Furthermore, mobile users can be directed to one of multiple secondary control channels by a primary control/pointer channel. Therefore, the receiver has to demodulate all possible secondary control channels, which results in even more hardware allocation requirements and the introduction of further processing delays (or increases in the real-time requirement if a software solution is used). Alternatively, the receiver has to generate and store all necessary codes (e.g., Walsh codes, short and long PN codes) and delay the demodulation of the secondary control channel until the primary control channels have been processed. Similarly, some of the data channel processing has to be delayed until all overhead channels are processed. Both of the previously mentioned alternatives significantly increase the implementation complexity, delay the demodulation of the data channels and also introduce further data channel processing delays.
FIG. 1 shows a partial block diagram of a prior art receiver for one-code channel. A typical prior art implementation for processing overhead control channels and multiple code data channels in a receiver is shown in FIG. 5. As shown, one demodulator is required for each control channel and data processing operations are performed on each control channel. For example, to implement one such prior art system, it would require a total of seven demodulators and seven overhead control channel data processing operations (e.g., one Dedicated Pointer Channel (DPTRCHs) and six Shared Control Channels (SHCCHs). One proposal for the 1xEVDV standard uses twenty-four Dedicated Pointer Channels (DPTRCH) and six Shared Control Channels (SHCCH). The pointer field in the DPTRCH directs the mobile unit to one of the six possible SHCCHs, and the SHCCH provides information, such as the Modulation and Coding Scheme (MCS) level, the Walsh code index, etc. for the receiver to decode the data channels. These two overhead channels are CDM with data traffic channels in the same time frame. Given the above, there exists a need in the art for a method and apparatus to process overhead control channels in a multiple code channel environment in a more efficient manner than has been accomplished by prior art techniques described above. For the case where one or multiple common control channels are used for demodulating the data channel, a similar requirement exists.